In a graph G = (V, E), a module is a vertex subset M of V such that every vertex outside M is adjacent to all or none of M . For example, ∅, {x} (x ∈ V ) and V are modules of G, called trivial modules. A graph, all the modules of which are trivial, is prime; otherwise, it is decomposable.there is some k-vertex set X of vertices such that there is no proper induced subgraph of G containing X is prime. From this perspective, I. Boudabbous proposes to find the (−k)-critical graphs and k-minimal graphs for some integer k even in a particular case of graphs. This research paper attempts to answer I. Boudabbous's question. First, it describes the (−k)-critical tree. As a corollary, we determine the number of nonisomorphic (−k)-critical tree with n vertices where k ∈ {1, 2, n 2 }. Second, it provide a complete characterization of the k-minimal tree. As a corollary, we determine the number of nonisomorphic k-minimal tree with n vertices where k ≤ 3.